As known in the art, use of a magnesium alloy, which is the most lightweight of currently useful metal materials, as a material for various metal components in lieu of aluminum is drastically increasing in order to achieve further weight reduction, and demand therefor is considerably increasing to address the issue of fuel efficiency of vehicles and aircraft and application to mobile electronic products.
A magnesium alloy has a density of 1.74 g/cc, which is the lightest among commercially available structural alloys, and the density thereof corresponds to ⅔ of the density of aluminum. Furthermore, a magnesium alloy has superior machinability, high vibration damping capacity, high ability to absorb vibrations and impact, and excellent electromagnetic wave-shielding performance. The reason why the application of a magnesium alloy to computers, mobile phones, components for vehicles, etc. has recently increased is that it is lightweight and has high recycling rate and the ability to shield electromagnetic waves, and casting thereof into a thin profile is possible because castability is superior to that of aluminum.
However, magnesium has a hexagonal close-packed lattice structure having not many slip system, which is essential for plastic deformation, and forming thereof is mainly performed through casting owing to poor extrudability or formability. Here, sand casting makes it difficult to form a desired shape, and die casting causes many problems in the subsequent surface treatment process because the cast structure thereof is porous. Then, materials such as AZ31, AM20 and the like are manufactured by alloying aluminum and zinc or manganese, whereby plastic working using the ductility of a single-phase solid solution becomes possible. However, these materials are developed so as to have a basal texture after annealing treatment, and a plate or a profile subjected to unidirectional plastic deformation has high anisotropy and makes it easy to form extension twins, and the commercialization thereof is delayed owing to problematic plastic working upon real-world application despite the high ductility thereof.
Since extrusion at a temperature of 400° C. or more is carried out in a temperature range in which a low-melting-point eutectic liquid phase and an alpha-magnesium solid solution co-exist due to frictional heat with a die, wrinkle-like defects, in which fine surface cracks resembling fingerprints appear, may occur. Such fine surface defects may decrease fatigue strength and thus must be removed. In actual fields, however, the removal thereof is not easy in terms of cost, environmental factors, and safety issues due to dust ignition. In order to obtain clean products having no surface defects, an extrusion process is performed at a temperature of 350° C. or less. To this end, the processing pressure has to be increased to at least five times 1000 kgf/cm2, which is the extrusion pressure of aluminum.
Moreover, conventional AZ- and AM-based magnesium alloys mainly used for a wrought product are problematic because flame retardancy is not assured due to a low-melting-point eutectic phase.
Such AZ- and AM-based magnesium alloys are disadvantageous in that copper (Cu) or high-melting-point iron-based impurities (Fe, Ni) having low solubility may form initial precipitates during the solidification thereof, and a beta-Mg17Al12 compound of aluminum and magnesium, which is subsequently precipitated, may form coarse plate-like precipitates, and such precipitates are thus interconnected and heat transfer is thus blocked, and thermal conductivity is remarkably lowered even by the addition of about 3 to 4% thereof (Ed. G. L. Song, Corrosion of Magnesium Alloys, 2011, pp. 137-146).
Thus, when a flame is applied to such a material, it is easy to drastically partially increase the temperature of the heated portion of the structure owing to its low thermal conductivity. When it is dissolved, it easily reacts with oxygen in the air and ignites, and even when the flame is extinguished, it is difficult to decrease the temperature of the material due to slow thermal diffusion, and thus combustion continues, making it difficult to achieve rapid extinguishment, and thus ensuring safety becomes impossible. The concern about fire affects not only vehicles but encompasses all industries, and the application thereof has thus been greatly delayed.
For this reason, many attempts have been made to add rare earth metals to magnesium in order to impart magnesium with flame retardancy.
Conventional materials therefor may include alloys such as WE43, ZE41, ZE10 or Elektron 21, containing rare earth elements such as yttrium, niobium (Nb), samarium (Sm), ytterbium (Yb), gadolinium (Gd), neodymium (Nd) and zirconium (Zr). These alloys manifest excellent flame retardancy due to a strong oxide film constituted by a rare earth element but require a large amount of expensive elements or have poor plastic workability, and thus do not adequately satisfy market requirements. When the rare earth element is contained in an amount of 4% or more, adverse effects in which ductility is remarkably decreased occur, and thermal conductivity is generally decreased with an increase in the amount of the alloy element that is added. In particular, zirconium functions to fine the grain size and to increase flame retardancy but has very low thermal conductivity and plastic workability. Hence, even when zirconium is added in an amount of about 1% to magnesium, thermal conductivity may be lowered by 50 to 70%.
As alloys containing zirconium, WE43 has thermal conductivity of 51 to 54 W/m·K and ZE41 has thermal conductivity of 24 W/m·K, and these are mainly used as casting materials, rather than plastic working materials, due to the low ductility thereof. Elektron 21 contains about 4% of a rare earth element and 0.5% or less of zinc and thus exhibits high thermal conductivity of 116 W/m·K and superior ignition suppression performance but very low elongation of about 2%, making it difficult to perform plastic working. ZE10, containing zirconium, has low thermal conductivity and plastic workability and has to be molded through a special molding process such as ECAP, making it difficult to actually use in plastic working applications in industrial sites.
Korean Patent No. 10-1367892 discloses a high-temperature magnesium alloy and a method of manufacturing the same, in which a magnesium alloy melt is added with 0.5 to 3.8% of calcium oxide (CaO), and aluminum and calcium are combined while calcium oxide is reduced, thus imparting flame retardancy. However, this alloy suffers from very low plastic workability.
Furthermore, a method of increasing thermal conductivity by mixing magnesium with silicon carbide (SiC) or fibrous alumina has been disclosed, but plastic workability is deteriorated, and thus the method is unsuitable for use in a tempering process (A. Rudajevova et al., On the Thermal Characteristics of Mg-Based Composites, Kompozyty, 4, 10, 2004).
Hence, the production of a magnesium alloy imparted with both flame retardancy and plastic workability is regarded as difficult.
Korean Patent No. 10-0509648, in which plastic workability is increased by the addition of a rare earth element, discloses a method of manufacturing a magnesium alloy plate having superior plastic workability from a magnesium alloy configured such that magnesium is added with zinc and yttrium. In this patent, a melt containing 0.5 to 5.0% of zinc and 0.2 to 2.0% of yttrium is cast into a plate-like material having a thickness of 35 mm, annealed and then rolled into a plate having a thickness of 1.0 mm, thus increasing the plastic workability of a rolled plate, but center segregation upon casting into billets having a large diameter of 75 mm or more and zinc gravitational segregation, which becomes severe when the zinc content is 3% or more, have not yet been overcome. Furthermore, this patent does not consider improvements in flame retardancy of the material at all.
In order to increase thermal conductivity and high-temperature stability of the material, a conventional AZ-based magnesium alloy is added with an alkaline earth metal such as strontium (Sr) or calcium and combined with beta-Mg17Al12 to adjust the shape thereof (A. Kielbus et al., The Thermal Diffusivity of Mg—Al—Sr and Mg—Al—Ca—Sr and Casting Magnesium Alloys, Defect and Diffusion Forum, Vol. 326-328, 2012, pp. 249-254). The strontium or calcium functions to increase the surface tension of a beta-phase precipitate to thus suppress the formation of the precipitate into a lamella phase at grain boundaries, and also, the size of the precipitate is decreased to thus improve thermal conductivity, and when the melt is exposed to flame, a dense surface oxide film is formed and ignition is thus prevented. However, this material is composed of 6 to 9% of aluminum with 0.8 to 2% of strontium or 1.5 to 2.2% of calcium, the total amount of alloy elements being 8 to 11%, whereby the resulting alloy has low ductility and is unsuitable for use in a tempering process. Furthermore, the thermal conductivity of this alloy is increased by about 75% compared to the thermal conductivity of AZ91, but is still only 87 W/m·K, similar to that of AZ31, and thus does not reach thermal conductivity of 100 W/m·K or more, which is desired in the present invention.
Korean Patent No. 10-1276665 discloses a magnesium alloy for plastic working, in which magnesium is added with 4 to 10% of tin and 0.05 to 1.0% of calcium, thus ensuring desired flame retardancy. However, in this patent, since a melt temperature has to be maintained at 850 to 900° C. in order to dissolve high-melting-point elements such as calcium, manganese, yttrium, erbium, etc., gas solid solubility and oxide content in the melt are unnecessarily increased, and thus the concentration of impurities is increased, and moreover, the likelihood of ignition of the melt is high, undesirably deteriorating working safety.
As an alternative thereto, Korean Patent No. 10-1406111 discloses a magnesium alloy composed mainly of magnesium and containing 6.5 to 7.5% of tin and 1% of each of zinc and aluminum.
These alloys are improved in flame retardancy but still exhibit low plastic workability and thermal conductivity due to the presence of a large amount of tin, having high precipitation hardenability, and thus, in order to extrude billets therefrom, thermal treatment for a long period of time at a high temperature of 480 to 500° C. and an extrusion pressure of 9946 kgf/cm2 are required, making it difficult to perform plastic working at a pressure of 1000 kgf/cm2 or less, which is a typical aluminum extruder pressure in the related industry.
Also, Korean Patent No. 10-0519721 discloses a high-strength magnesium alloy composed basically of magnesium and 6% of zinc and further comprising 0.4 to 3% of manganese, aluminum, silicon or calcium. However, when this alloy is manufactured into commercially available billets, the large amount of zinc may cause gravitational segregation, and thus billets may break down during extrusion, or plastic workability may decrease, and only high strength and plastic workability are mentioned in the detailed description thereof, and no grounds for expecting good performance in flame retardancy or thermal conductivity are found therein.
In this way, the magnesium alloy has been developed in terms only of plastic workability or flame retardancy at an early stage, but for actual commercialization thereof, both thermal conductivity and flame retardancy should be satisfied and plastic workability also has to be ensured. In order to commercialize the structural material, simply satisfying only strength and moldability is insufficient, and ignition of the magnesium material in the event of a fire should be inhibited in order to prevent the spread of fire and ensure safety.
With the goal of promoting this delayed technological development, the Federal Aviation Administration (FAA), U.S.A., realistically revised the standards for flammability testing of magnesium alloys for aircraft seat structures.
According to this regulation (DOT/FAA/TC-13/52) amended in 2014, unless the initial weight is reduced by 10% or more in a flammability test for a total of 7 min, in which ignition should not occur within 2 min when exposed to an oil burner flame for 4 min (240 sec) and in which self-extinguishment should occur within 3 min after the burner is turned off, the magnesium alloy meets the performance standard.
FIG. 1 schematically shows a flammability tester for a magnesium alloy for use in an aircraft approved by the FAA and a test specimen.